Doctoral Dissertations

Date of Award

12-1986

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

Walter Frost

Committee Members

John Caruthers, Frank Collins, Remi Engels, Kenneth R. Kimble, Bharat Soni, Robert E. Turner

Abstract

The computer-simulated turbulence model plays an important role in the study of diffusive behavior of turbulent flow fields. Diffusion models are used to investigate certain physical phenomena. These models are continuously being developed to include additional features. The model presented in this study is to provide the non-zero cross-spectrum effect and the large-scale eddy structure of the atmospheric boundary layer turbulence.

In simulating turbulence fluctuations, a computer-generated random signal is passed through a digital filter. The filter is designed to produce an output signal having the same energy spectrum as the turbulence observed in the atmospheric boundary layer. Atmospheric turbulence shows that a non-zero cross-spectrum exists in the planetary boundary layer between the longitudinal and the vertical components of turbulence at low frequencies. In this study a technique to account for the non-zero cross-spectrum is considered.

The mechanism for large-scale eddy structure is convection. Thermal plumes exist in the unstable atmosphere. The plumes are effective in carrying warm air to the top of the convective layer. In boundary layers, large eddies are elongated longitudinal vortices, and they are associated with the low-frequency end of the energy spectrum. To consider the effect of large eddies in the turbulence simulation, an energy spectrum expression representative of the low-frequency portion of the spectrum as well as the high-frequency portion is employed.

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